The invention relates to a receiver for angle-modulated carrier signals, particularly of the tamed FM-type, the receiver comprising an input circuit for deriving the transmitted angle-modulated carrier signal from a transmission channel, a reference carrier circuit, a demodulation circuit coupled to the input circuit for demodulating the transmitted angle-modulated carrier signal with the reference carriers for generating first and second demodulated signals, the receiver further comprising a decision circuit to obtain regenerated binary data signals from the first and second demodulated signals.
In radio communication systems, use is preferably made of modulation methods which result in a modulated carrier signal of a (substantially) constant amplitude (envelope), which means the use of angle modulation. See, for example, F. de Jager, C. B. Dekker, IEEE Trans, Comm., Vol. CDM-26, No. 5, May 1978, pages 534-542 for the use of "tamed FM" (TFM) for that purpose. It has been found that a receiver for TFM signals has an advantageous signal-to-noise ratio versus the error rate performance (expressed in BER: bit error rate) if use is made of coherent demodulation.
Specifically, in mobile radio communication it often occurs that the received signal disappears as a result of, for example, fading. The circuit in the receiver in which the carrier required for coherent detection is regenerated, has a certain acquisition time: correct reception is not possible until approximately 50 bits have been received. After each fade, acquisition will be required again which results in loss of information. In secrecy radio communication systems "frequency hopping" is frequently used: during the communication the carrier frequency is changed in order to hinder unwanted overhearing. Also in that case, after adjusting to a different carrier frequency, loss of information will occur owing to the renewed acquisition.